Ultrafast carrier dynamics in PbS quantum dots (QDs) is characterized using a transient grating technique. In addition to an initial peak, a new peak appears at about 3 ps when the excitation photon energy is greater than 2.7E(g) (E-g: band gap between LUMO and HOMO in the QDs). The first peak is considered to be related to hot carriers and the second one is believed to result from multiple exciton generation (MEG). We found that MEG began in as fast as 200 fs and complete around 3 ps. We succeeded in observing the dynamics of MEG occurrence in PbS QDs. (C) 2012 Elsevier B. V. All rights reserved.